In recent years, fuel batteries which use hydrogen as fuel gas and oxygen as oxidant gas to generate electric energy from an electrochemical reaction have been developed and applied to vehicles, vessels, small power generators, and so on. Fuel batteries used for these applications not only generate electric energy from an electrochemical reaction but also produce reaction water. Most of the produced reaction water is discharged outside a fuel battery, but the reaction water partially remains within a fuel battery. In particular, as vehicles may be used in a low temperature environment falling below zero, a problem may occur in that the power generation of a fuel battery is disabled because the residual reaction water in the fuel battery is frozen preventing reaction gas from reaching the anode and cathode of the fuel battery.
In order to resolve such a problem, in JP 2007-149572 A (Patent Document 1), when the power generation is stopped, undesired water within a fuel battery is removed by a scavenging operation executed by an air compressor. More specifically, Patent Document 1 discloses techniques for scavenging water within a fuel battery cell by indirectly obtaining the amount of water within the fuel battery cell from measurement of internal resistance using an AC impedance method to determine when to stop scavenging.
The internal resistance of a fuel battery is known to be influenced by a humidity level of internal electrolyte film. When the amount of water within a fuel battery is small and the internal electrolyte film is dry, the internal resistance increases and the output voltage of the fuel battery is decreased. On the other hand, when the amount of water within a fuel battery is excessive, the anode and cathode of the fuel battery is covered with a water film, similarly resulting in a decreased output voltage of the fuel battery. Therefore, the management of the amount of water within a fuel battery is important.    Patent Document 1: JP 2007-149572 A